Parts with contours having curves and bends are cut out from material, for example, steel plate, stainless steel plate, or nonferrous metal plate such as aluminium plate, plastic plate such as acrylic resin plate, or plywood, by using a torch such as a gas cutting torch, a plasma cutting torch or a laser cutting torch.
There are the following methods for continuously controlling the movements of torch: (i) a numerical controlling method in which the position of a torch is designated on an orthogonal coordinate system and the path of the torch directed from the present point to the second point is given by a straight line or arc on a orthogonal coordinate system; (ii) an optical tracing method in which the reference point corresponding to the torch is set on a given portion of a figure, the direction from the reference point to a portion of the line or edge of the figure positioned at a distance from the reference point is detected, and the torch is moved in the detected direction. The latter method is low-cost and handy. Further, in the latter method, a torch can be controlled at a relatively high speed. Therefore, the optical tracing method is widespread.
A typical tracer is disclosed in Japanese patent publication of application No. 16916 of 1974. This tracer, as shown in FIG. 6, comprises a lens 52 rigidly fixed on a housing 51, a photocell 53 mounted on the surface of the lens 52 coaxially with the center of the lens 52, a shaft 55 rotated by a motor 54 mounted on the axis of the lens 52, a generator 56 which has two rotors 56a, 56b rigidly fixed on the shaft 55 at a right angle to each other, and a mirror 58 disposed eccentrically to the axis of the motor 54, and on which an image of a portion of a FIG. 57 on a drawing paper is formed.
The lens 52 is disposed above the FIG. 57, and the motor 54 is driven to rotate the shaft 55, while the mirror 58 scans a point 0 at distance from the projective point 0 of the axis of the lens 52, which position depends on an interval between the mirror 58 and the axis of the lens 52 and the tilting angle of the mirror 58. While the mirror 58 scans the edge of the FIG. 57, output of the photocell 53 changes. (Outputs in two orthogonal directions from the generator 56 are detected simultaneously with a change in the output of the photocell 53. The housing 51 is moved according to the detected output from the generator 56 by which the edge of the FIG. 57 can be traced. Accordingly, the edge of the FIG. 57 can be traced by moving the means for moving the housing 51, together with the means for holding a torch or by moving the means for holding a torch according to the output of the generator 56 so that parts with the same contour as that of the FIG. 57 can be cut out from material.
In the above-mentioned tracer, the figure is pre-read, then the edge of the figure is traced. Therefore, in case the figure has a corner with a relatively small radius of curvature, a portion of the contour of parts corresponding to the corner in the figure is formed into a curve with a big radius of curvature.
That is, as shown in FIGS. 6,7, when the lens 52 is disposed at a distance from the FIG. 57, the distance between the projective point 0 of the axis of the lens on the figure and the scanning point 0 of the mirror which is set according to the preset tilting angle of the mirror 58 and the distance between the mirror and the axis of the lens 52 is set as pre-reading amount L. Then, when the scanning point 0 is moved along the edge of the FIG. 57 having the corner with a relatively small radius of curvature, outputs of two orthogonal components from the generator 56 according to the direction from the projective point 0 to the scanning point 0 are sent to the means for moving a torch so that the torch is moved passing the projective points 0 without passing the corner in the figure with the result that the torch moves in a curved path with a big radius of curvature.
In the above-mentioned tracer, when the pre-reading amount L is increased so that the figure can be traced at a high speed and steadily, the radius of curvature of the path of the torch corresponding to the corner in the figure is increased so that tracing accuracy is decreased. On the other hand, when the pre-reading amount L is decreased so that the tracing accuracy is increased, it becomes impossible to trace the figure at the high speed.
Further, in order to increase the pre-reading amount L with a view to increasing the tracing accuracy, it is necessary to decrease the tilting angle of the mirror 58 against the axis of the lens 52, or to alter the focal length of the lens 52 together with bringing the mirror close to the lens 52. However, since the tracer is precision equipment, such a alteration is impossible, in fact, without changing the tracer itself.